New applications of measurement and imaging systems that employ tunable light sources demand increasingly higher data acquisition rates. For example, although the latest generation of frequency-domain optical coherence tomography (OCT) systems for imaging of coronary arteries acquire image data at 50,000 lines/sec, even faster speeds are necessary to scan the full lengths of the major coronary branches. Scanning spectrometers must also operate at higher speeds to permit rapid analysis of atmospheric aerosols and contaminants over large areas, hyperspectral imaging of moving targets, detection of chemical substances in multiple samples, and other applications.
One of the major impediments to higher speed operation of frequency-domain systems, especially systems that operate continuously over a wide spectral range, is the sweep repetition rate of the light source. Many high-speed wavelength-swept light sources employ mechanically actuated filters that are excited by a substantially periodic waveform with a frequency close to the resonance of the filter. When operated at very high speeds, many tunable light sources produce a usable output over less than 50% of the sweep period of the filter. This occurs because the time required to build up stimulated emission limits the available optical gain within the filter bandpass. Even if the build-up time of the gain element does not limit the scanning speed for a given application, the non-linearities of the gain element can broaden the line width of the source. Such undesirable broadening can occur during either the up sweep (the portion of the sweep over which the wavelength increases) or the down sweep (the portion of the sweep over which the wavelength decreases), such that a significant portion of the sweep becomes unusable. Moreover, to ease the demands on the data acquisition system, it is often desirable to acquire data only during the portion of the sweep when the rate of change of the optical frequency is approximately constant. These factors impose an upper limit on the effective duty cycle of continuously swept light sources.
A method for increasing the repetition rate of a light source using time-multiplexing was first introduced in the context of the design of Fourier-domain mode-locked lasers. This method, referred to here as ‘optical buffering’, is based on combining the output of the swept laser with a delayed version of the swept laser output, which is inserted within the interval between laser sweeps during the time when the laser gain medium is turned off or is inactive.
In a conventional buffer, the delayed version of the laser output is generated by splitting off a fixed fraction (˜50%) of the laser output with an optical coupler, storing the laser output in a spool of fiber and then recombining the original and delayed laser outputs using a second optical coupler. The length of the fiber is given by L=ncτ, where τ is the desired delay time, c is the speed of light and n is the refractive index of the fiber. Using such an optical buffering apparatus, the effective repetition rate of a laser with an original duty cycle less than 50% can be doubled. A second optical buffering stage can be added to quadruple the duty cycle of a swept laser with an original duty cycle less than 25%.
With respect to their commercial use in imaging or spectroscopy systems, the optical buffering configurations that have been disclosed previously need to employ a polarization controller to align the polarization states of the original and delayed versions of the laser output if a consistent sweep-to-sweep polarization state is desired. One polarization controller is required for each stage of buffering. To compensate for environmental effects on the birefringence of the spooled optical fiber, a means of adjusting each polarization controller continuously to maintain alignment of the polarization states is required. These requirements increase system cost and complexity.
Additionally, the optical throughput of such conventional optical buffer configurations is relatively low. For example, the optical loss of a conventional buffer, configured for doubling the laser repetition rate, exceeds 3 dB, because at least one-half of the light is lost in the single-mode coupler that combines the original and delayed laser outputs. In certain applications, an optical power loss of this magnitude can negatively impact a system's signal-to-noise ratio.
The present invention addresses these issues.